1. Field of the Invention
The present invention relates to an array waveguide grating and a manufacturing method of the same.
2. Description of the Related Art
FIG. 6 of the accompanying drawings is a plan view showing an example of conventional array waveguide gratings.
The array waveguide grating 1 shown in this figure is includes a substrate 2, an input channel waveguide 3 formed on top of the substrate 2 for accepting wavelength-multiplexed signal light at one end thereof (in the figure, the left end), an input-side fan-shaped slab waveguide 4 connected to the input channel waveguide 3 at one end thereof, an array of a plurality of channel waveguides 5, one end of which array is connected to the other end (in the figure, the right end) of the input-side fan-shaped slab waveguide 4 such that lengths of the channel waveguides 5 become successively longer by a desired variation, an output-side fan-shaped slab waveguide 6, one end of which is connected to the other ends of the array channel waveguides 5, and a plurality of output channel waveguides 7, each of which has one end connected to the other end of the output side fan-shaped slab waveguide 6 for outputting signal lights which have been split.
Signal light of various wavelengths (xcex1, xcex2, . . . , xcexn) introduced to the input channel waveguide 3 is spread out in the input-side slab waveguide 4 by diffraction because it is not trapped in the transverse direction (surface direction of the substrate 2) in the input-side fan-shaped slab waveguide 4, and is propagated along the array of channel waveguides 5. In this array 5, the channel waveguides extend different lengths (successively elongated by a desired length) so that equiphase planes of the signal light are inclined in accordance with the wavelengths of the signal light. Thus, the points of convergence of the signal light shift at the connecting face between the output-side fan-shaped slab waveguide 6 and the output channel waveguides 7.
Therefore, the signal light of different wavelengths is incident on the output channel waveguides 7 respectively, and the signal light of different wavelengths (xcex1, xcex2, . . . , xcexn) is outputted from the output ends of the output channel waveguides 7 respectively.
FIGS. 7A and 7B are diagrams showing the relationship between converging beam fields of the output-side fan-shaped slab waveguide and fields of the output channel waveguides of the array waveguide grating shown in FIG. 6. FIGS. 8A and 8B are diagrams of wavelength-loss characteristics of the array waveguide grating 1 shown in FIG. 6, in which the horizontal axis indicates wavelength and the vertical axis indicates loss.
The loss of the array waveguide grating 1 is proportional to the overlap integral (hatched portion) of the electric field distribution xcfx86f of the converging beam and the electric field distribution xcfx86o of the output waveguides as shown in FIGS. 7A and 7B. Because the electric field of the output channel waveguide 7 has a single-peaked distribution, the loss is lowest at the wavelength that matches the peak of the converging beam xcfx86f, making for a peaky wavelength-loss characteristic (FIG. 8A).
Accordingly, a waveguide structure that uses Y-branching or a parabolic shape is employed such that the electric field distribution of the input channel waveguide 3 (or the output channel waveguide 7) has a dual-peaked shape as shown in FIG. 7B. In this case, even if the points of convergence of the beams shift to a certain extent, the value of the overlap integral hardly changes, making for a flat wavelength-loss characteristic (FIG. 8B).
FIG. 9 is an enlarged view of part of the wavelength-loss characteristic of the array waveguide grating shown in FIG. 6. The horizontal axis indicates the wavelength, and the vertical axis indicates the loss.
When the electric field distribution of the output waveguide (or the input waveguide) has a dual-peaked shape, it is difficult to achieve total flatness in the wavelength-loss characteristics distribution. Further, 2 to 3 dB of additional loss is generated to realize the flatness. When application in a wavelength-multiplexed optical telecommunications system is considered, this loss increase is a problem and loss improvement must be made. When a yet flatter wavelength-loss characteristics is sought in this structure, another problem arises; a ripple appears in the transmission region as shown in FIG. 9, greatly affecting transmission characteristics.
Accordingly, an object of the present invention is to provide an array waveguide grating, which solves the above-mentioned problems, and which has approximately flat wavelength-loss characteristics in the transmission region without causing an increase in loss, and a manufacturing method of the array waveguide grating.
An array waveguide grating of the present invention comprises a substrate, an input channel waveguide formed on top of the substrate for receiving wavelength-multiplexed signal light at one end thereof; a plurality of output channel waveguides disposed parallel to the input channel waveguide for outputting split output signal light from respective one ends thereof; a fan-shaped slab waveguide connected to the other end of the input channel waveguide at one end thereof and to the other ends of the output channel waveguides at the same one end thereof, an array of a plurality of channel waveguides connected to the other end of the fan-shaped slab waveguide at one end thereof such that lengths of the channel waveguides are successively elongated by a desired variation xcex94L, and an array of Fabry-Perot resonators connected to the other end of the array of channel waveguides at one end thereof such that lengths of the resonators are successively elongated by twice the channel waveguide length variation xcex94L.
The signal light obtained by splitting the wavelength-multiplexed signal light in the array of channel waveguides is reflected inside the Fabry-Perot resonators, and this split signal light is caused to pass through the array of channel waveguides once again. Therefore, the array waveguide grating provided by the invention has approximately flat wavelength-loss characteristics in the transmission region without increasing loss.
Preferably a reflectance of the Fabry-Perot resonator array at its face connecting with the channel waveguide array is approximately 11%, and a reflectance at its face opposite the connecting face is approximately 100% (full reflection).
The face of the Fabry-Perot resonator array in contact with the channel waveguide array may be coated with a metallic film having a reflectance of approximately 11%, and the opposite face may be coated with either a metallic film having a reflectance of approximately 100%, or a multilayer dielectric film of TiO2 and SiO2 having a reflectance of approximately 100%.
The Fabry-Perot resonator array may be constituted from an array of semicircular concentric channel waveguides, respective one ends of which are connected to the other ends of the array channel waveguides such that lengths of the semicircular concentric channel waveguides become successively longer by twice the waveguide length variation xcex94L, a reflective film formed between the channel waveguide array and semicircular channel waveguide array and having a reflectance of approximately 11%, and another reflective film formed at the other end of the semicircular channel waveguide array and having a reflectance of approximately 100%.
Alternatively the Fabry-Perot resonator array may be constituted from a second array of channel waveguides, respective one ends of which are connected to the other ends of the first array of channel waveguides such that lengths of the second channel waveguides are successively elongated by twice the first waveguide length variation xcex94L, a first groove transversely crossing the first and second channel waveguide arrays at the connecting portion of these arrays, a reflective film fitted in the first groove and having a reflectance of approximately 11%, a second groove transversely extending across the second channel waveguide array midway, and another reflective film fitted in the second groove and having a reflectance of approximately 100%.
The reflective film of the array waveguide grating may be formed by vapor deposition of a metallic film of silicon, gold or similar, on top of a thin film of polyimide.
In the array waveguide grating, preferably the polyimide reflective film, on which the metallic film has been deposited by vapor deposition, is affixed using optical resin.
In the array waveguide grating, preferably the channel waveguides are formed either on top of a quartz substrate or a silicon substrate, and constituted by a material having quartz glass as a principal constituent.
Preferably the channel waveguides are primarily made from InP.
A method of manufacturing an array waveguide grating according to the present invention includes the steps of forming, in parallel on top of a substrate, an input channel waveguide adapted to receive wavelength-multiplexed signal light at one end thereof, and a plurality of output channel waveguides adapted to output split output signal light from one end thereof forming a fan-shaped slab waveguide on the same substrate such that one end thereof is connected to the other end of the input channel waveguide and the other ends of the output channel waveguides; forming at the other end of the fan-shaped slab waveguide on the same substrate an array of a plurality of channel waveguides, which becomes successively longer by a desired waveguide length variation xcex94L; forming, on top of a second substrate, an array of Fabry-Perot resonators which become successively longer by twice the waveguide length variation xcex94L; and connecting the first substrate to the second substrate such that one end of the Fabry-Perot resonator array connects to the other end of the channel waveguide array.
In the manufacturing method for the array waveguide grating according to the present invention, the Fabry-Perot resonator array may be constituted from an array of semicircular concentric channel waveguides formed on top of the second substrate such that lengths of the semicircular concentric channel waveguides become successively longer by twice the waveguide length variation xcex94L, a reflective film formed at one end of the array of semicircular concentric channel waveguides and having a reflectance of approximately 11%, and another reflective film formed at the other end of the array of semicircular concentric channel waveguides and having a reflectance of approximately 100%, and the first substrate may be connected to the second substrate such that the reflective film having a reflectance of approximately 11% is inserted between the other end of the array of channel waveguides and the one end of the array of the semicircular concentric channel waveguides.
Alternatively, in the manufacturing method for the array waveguide grating according to the present invention, the Fabry-Perot resonator array may be constituted from a second array of channel waveguides which are formed on the same substrate such that lengths of these channel waveguides are successively elongated by twice the waveguide length variation xcex94L and such that respective one ends thereof are connected to the other ends of the first array of channel waveguides, a first groove intersecting with both the first and second arrays of channel waveguides at a connecting portion of the first and second arrays of channel waveguides, a reflective film having a reflectance of approximately 11% and fitted in the first groove, a second groove intersecting with the second array of channel waveguides midway, and another reflective film having a reflectance of approximately 100% and fitted in the second groove.